The FDA and medical field strongly differentiate between the two in terms of risk.
So much so that even women who have had sexually contact with a male who has had sexual contact with other men are deferred from donating blood for a year afterward, I believe.
The key issue likely is how being sexually active with someone is in a medical sense the same as being sexually active with everyone else that person has been sexually active with.
And this:
“Sex with an HIV-positive partner was associated with a 132-fold increase in risk (multivariable adjusted odds ratio) for being HIV-positive, and a history of male-to-male sexual contact was associated with a 62-fold increase in risk.
“By comparison, the increase in risk for a history of multiple sexual partners of the opposite sex in the last year was 2.3-fold.”
“As a group, in the United States, MSM have the highest HIV risk: according to CDC, two-thirds of new HIV infections occur in the approximately 2% of the population who are MSM (Ref. 27). The risk of HIV among MSM is more than twenty-fold higher than that of men who have sex with multiple female partners and women who have sex with multiple male partners (Ref. 32).”
http://www.freerepublic.com/focus/f-news/3440651/posts
And the questions asked of blood donors:
http://www.aabb.org/tm/questionnaires/Documents/dhq/v2/DHQ%20v2.0.pdf
That info is very helpful. Intuitively I would have never come up with that conclusion. So they exclude from the blood supply on the front end those donations which when tested would almost always prove defective.